How to use and configure the API client

Overview

The PrefectClient offers methods to simplify common operations against Prefect’s REST API that may not be abstracted away by the SDK. For example, to reschedule flow runs, one might use methods like:

read_flow_runs with a FlowRunFilter to read certain flow runs
create_flow_run_from_deployment to schedule new flow runs
delete_flow_run to delete a very Late flow run

Getting a client

By default, get_client() returns an asynchronous client to be used as a context manager, but you may also use a synchronous client.

from prefect import get_client

async with get_client() as client:
    response = await client.hello()
    print(response.json()) # 👋

You can also use a synchronous client:

from prefect import get_client

with get_client(sync_client=True) as client:
    response = client.hello()
    print(response.json()) # 👋

Pagination and default query limits

Client methods that accept limit and offset parameters - such as read_flow_runs, read_deployments, and read_task_runs — are subject to a server-side maximum. When limit is None (the default), the server applies PREFECT_API_DEFAULT_LIMIT, which defaults to 200. To retrieve all matching records, paginate with offset:

from prefect import get_client


async def read_all_deployments(page_size: int = 200):
    all_deployments = []
    offset = 0

    async with get_client() as client:
        while True:
            page = await client.read_deployments(
                limit=page_size, offset=offset
            )
            if not page:
                break
            all_deployments.extend(page)
            if len(page) < page_size:
                break
            offset += page_size

    return all_deployments

Configure custom headers

You can configure custom HTTP headers to be sent with every API request by setting the PREFECT_CLIENT_CUSTOM_HEADERS setting. This is useful for adding authentication headers, API keys, or other custom headers required by proxies, CDNs, or security systems.

Setting custom headers

Custom headers can be configured via environment variables or settings. The headers are specified as key-value pairs in JSON format.

export PREFECT_CLIENT_CUSTOM_HEADERS='{"CF-Access-Client-Id": "your-client-id", "CF-Access-Client-Secret": "your-secret"}'

prefect config set PREFECT_CLIENT_CUSTOM_HEADERS='{"CF-Access-Client-Id": "your-client-id", "CF-Access-Client-Secret": "your-secret"}'

[client]
custom_headers = '''{
    "CF-Access-Client-Id": "your-client-id",
    "CF-Access-Client-Secret": "your-secret",
    "X-API-Key": "your-api-key"
}'''

Protected headersCertain headers are protected and cannot be overridden by custom headers for security reasons:

User-Agent - Managed by Prefect to identify client version
Prefect-Csrf-Token - Used for CSRF protection
Prefect-Csrf-Client - Used for CSRF protection

If you attempt to override these headers, Prefect will log a warning and ignore the custom header value.

Examples

These examples are meant to illustrate how one might develop their own utilities for interacting with the API.

If you believe a client method is missing, or you’d like to see a specific pattern better represented in the SDK generally, please open an issue.

Reschedule late flow runs

To bulk reschedule flow runs that are late, delete the late flow runs and create new ones in a Scheduled state with a delay. This is useful if you accidentally scheduled many flow runs of a deployment to an inactive work pool, for example. The following example reschedules the last three late flow runs of a deployment named healthcheck-storage-test to run six hours later than their original expected start time. It also deletes any remaining late flow runs of that deployment. First, define the rescheduling function:

async def reschedule_late_flow_runs(
    deployment_name: str,
    delay: timedelta,
    most_recent_n: int,
    delete_remaining: bool = True,
    states: list[str] | None = None
) -> list[FlowRun]:
    states = states or ["Late"]

    async with get_client() as client:
        flow_runs = await client.read_flow_runs(
            flow_run_filter=FlowRunFilter(
                state=dict(name=dict(any_=states)),
                expected_start_time=dict(before_=datetime.now(timezone.utc)),
            ),
            deployment_filter=DeploymentFilter(name={'like_': deployment_name}),
            sort=FlowRunSort.START_TIME_DESC,
            limit=most_recent_n if not delete_remaining else None
        )

        rescheduled_flow_runs: list[FlowRun] = []
        for i, run in enumerate(flow_runs):
            await client.delete_flow_run(flow_run_id=run.id)
            if i < most_recent_n:
                new_run = await client.create_flow_run_from_deployment(
                    deployment_id=run.deployment_id,
                    state=Scheduled(scheduled_time=run.expected_start_time + delay),
                )
                rescheduled_flow_runs.append(new_run)
            
        return rescheduled_flow_runs

Then use it to reschedule flows:

rescheduled_flow_runs = asyncio.run(
    reschedule_late_flow_runs(
        deployment_name="healthcheck-storage-test",
        delay=timedelta(hours=6),
        most_recent_n=3,
    )
)

View the complete example

reschedule_late_flows.py

from __future__ import annotations

import asyncio
from datetime import datetime, timedelta, timezone

from prefect import get_client
from prefect.client.schemas.filters import DeploymentFilter, FlowRunFilter
from prefect.client.schemas.objects import FlowRun
from prefect.client.schemas.sorting import FlowRunSort
from prefect.states import Scheduled

async def reschedule_late_flow_runs(
    deployment_name: str,
    delay: timedelta,
    most_recent_n: int,
    delete_remaining: bool = True,
    states: list[str] | None = None
) -> list[FlowRun]:
    states = states or ["Late"]

    async with get_client() as client:
        flow_runs = await client.read_flow_runs(
            flow_run_filter=FlowRunFilter(
                state=dict(name=dict(any_=states)),
                expected_start_time=dict(before_=datetime.now(timezone.utc)),
            ),
            deployment_filter=DeploymentFilter(name={'like_': deployment_name}),
            sort=FlowRunSort.START_TIME_DESC,
            limit=most_recent_n if not delete_remaining else None
        )

        if not flow_runs:
            print(f"No flow runs found in states: {states!r}")
            return []
        
        rescheduled_flow_runs: list[FlowRun] = []
        for i, run in enumerate(flow_runs):
            await client.delete_flow_run(flow_run_id=run.id)
            if i < most_recent_n:
                new_run = await client.create_flow_run_from_deployment(
                    deployment_id=run.deployment_id,
                    state=Scheduled(scheduled_time=run.expected_start_time + delay),
                )
                rescheduled_flow_runs.append(new_run)
            
        return rescheduled_flow_runs


if __name__ == "__main__":
    rescheduled_flow_runs = asyncio.run(
        reschedule_late_flow_runs(
            deployment_name="healthcheck-storage-test",
            delay=timedelta(hours=6),
            most_recent_n=3,
        )
    )
    
    print(f"Rescheduled {len(rescheduled_flow_runs)} flow runs")
    
    assert all(run.state.is_scheduled() for run in rescheduled_flow_runs)
    assert all(
        run.expected_start_time > datetime.now(timezone.utc)
        for run in rescheduled_flow_runs
    )

Get the last `N` completed flow runs from your workspace

To get the last N completed flow runs from your workspace, use read_flow_runs and prefect.client.schemas. This example gets the last three completed flow runs from your workspace:

async def get_most_recent_flow_runs(
    n: int,
    states: list[str] | None = None
) -> list[FlowRun]:    
    async with get_client() as client:
        return await client.read_flow_runs(
            flow_run_filter=FlowRunFilter(
                state={'type': {'any_': states or ["COMPLETED"]}}
            ),
            sort=FlowRunSort.END_TIME_DESC,
            limit=n,
        )

Use it to get the last 3 completed runs:

flow_runs: list[FlowRun] = asyncio.run(
    get_most_recent_flow_runs(n=3)
)

View the complete example

get_recent_flows.py

from __future__ import annotations

import asyncio

from prefect import get_client
from prefect.client.schemas.filters import FlowRunFilter
from prefect.client.schemas.objects import FlowRun
from prefect.client.schemas.sorting import FlowRunSort

async def get_most_recent_flow_runs(
    n: int,
    states: list[str] | None = None
) -> list[FlowRun]:    
    async with get_client() as client:
        return await client.read_flow_runs(
            flow_run_filter=FlowRunFilter(
                state={'type': {'any_': states or ["COMPLETED"]}}
            ),
            sort=FlowRunSort.END_TIME_DESC,
            limit=n,
        )


if __name__ == "__main__":
    flow_runs: list[FlowRun] = asyncio.run(
        get_most_recent_flow_runs(n=3)
    )
    assert len(flow_runs) == 3
    
    assert all(
        run.state.is_completed() for run in flow_runs
    )
    assert (
        end_times := [run.end_time for run in flow_runs]
    ) == sorted(end_times, reverse=True)

Instead of the last three from the whole workspace, you can also use the DeploymentFilter to get the last three completed flow runs of a specific deployment.

Transition all running flows to cancelled through the Client

Use get_clientto set multiple runs to a Cancelled state. This example cancels all flow runs that are in Pending, Running, Scheduled, or Late states when the script is run.

async def list_flow_runs_with_states(states: list[str]) -> list[FlowRun]:
    async with get_client() as client:
        return await client.read_flow_runs(
            flow_run_filter=FlowRunFilter(
                state=FlowRunFilterState(
                    name=FlowRunFilterStateName(any_=states)
                )
            )
        )

async def cancel_flow_runs(flow_runs: list[FlowRun]):
    async with get_client() as client:
        for flow_run in flow_runs:
            state = flow_run.state.copy(
                update={"name": "Cancelled", "type": StateType.CANCELLED}
            )
            await client.set_flow_run_state(flow_run.id, state, force=True)

Cancel all pending, running, scheduled or late flows:

async def bulk_cancel_flow_runs():
    states = ["Pending", "Running", "Scheduled", "Late"]
    flow_runs = await list_flow_runs_with_states(states)

    while flow_runs:
        print(f"Cancelling {len(flow_runs)} flow runs")
        await cancel_flow_runs(flow_runs)
        flow_runs = await list_flow_runs_with_states(states)

asyncio.run(bulk_cancel_flow_runs())

View the complete example

cancel_flows.py

import asyncio

from prefect import get_client
from prefect.client.schemas.filters import FlowRunFilter, FlowRunFilterState, FlowRunFilterStateName
from prefect.client.schemas.objects import FlowRun, StateType

async def list_flow_runs_with_states(states: list[str]) -> list[FlowRun]:
    async with get_client() as client:
        return await client.read_flow_runs(
            flow_run_filter=FlowRunFilter(
                state=FlowRunFilterState(
                    name=FlowRunFilterStateName(any_=states)
                )
            )
        )


async def cancel_flow_runs(flow_runs: list[FlowRun]):
    async with get_client() as client:
        for idx, flow_run in enumerate(flow_runs):
            print(f"[{idx + 1}] Cancelling flow run '{flow_run.name}' with ID '{flow_run.id}'")
            state_updates: dict[str, str] = {}
            state_updates.setdefault("name", "Cancelled")
            state_updates.setdefault("type", StateType.CANCELLED)
            state = flow_run.state.copy(update=state_updates)
            await client.set_flow_run_state(flow_run.id, state, force=True)


async def bulk_cancel_flow_runs():
    states = ["Pending", "Running", "Scheduled", "Late"]
    flow_runs = await list_flow_runs_with_states(states)

    while len(flow_runs) > 0:
        print(f"Cancelling {len(flow_runs)} flow runs\n")
        await cancel_flow_runs(flow_runs)
        flow_runs = await list_flow_runs_with_states(states)
    print("Done!")


if __name__ == "__main__":
    asyncio.run(bulk_cancel_flow_runs())

Query events with pagination

Query historical events from the Prefect API with support for filtering and pagination. This is useful for analyzing past activity, debugging issues, or building custom monitoring tools. The following example queries events from the last hour and demonstrates how to paginate through results:

from datetime import datetime, timedelta, timezone

from prefect import get_client
from prefect.events.filters import EventFilter, EventOccurredFilter


async def query_recent_events():
    async with get_client() as client:
        # query events from the last hour
        now = datetime.now(timezone.utc)
        event_filter = EventFilter(
            occurred=EventOccurredFilter(
                since=now - timedelta(hours=1),
                until=now,
            )
        )

        # get first page
        page = await client.read_events(filter=event_filter, limit=10)
        print(f"Total events: {page.total}")

        # iterate through all pages
        while page:
            for event in page.events:
                print(f"{event.occurred} - {event.event}")
            page = await page.get_next_page(client)

View the complete example

query_events.py

import asyncio
from datetime import datetime, timedelta, timezone

from prefect import get_client
from prefect.events.filters import EventFilter, EventOccurredFilter


async def query_recent_events():
    async with get_client() as client:
        # query events from the last hour
        now = datetime.now(timezone.utc)
        event_filter = EventFilter(
            occurred=EventOccurredFilter(
                since=now - timedelta(hours=1),
                until=now,
            )
        )

        # get first page with small limit to demonstrate pagination
        print("=== first page ===")
        event_page = await client.read_events(filter=event_filter, limit=5)
        print(f"total events: {event_page.total}")
        print(f"events on this page: {len(event_page.events)}")
        for event in event_page.events:
            print(f"  {event.occurred} - {event.event}")
        print()

        # if there are more pages, fetch the next one
        second_page = await event_page.get_next_page(client)
        if second_page:
            print("=== second page ===")
            print(f"events on this page: {len(second_page.events)}")
            for event in second_page.events:
                print(f"  {event.occurred} - {event.event}")
            print()

        # demonstrate iterating through all pages
        print("=== collecting all events ===")
        all_events = []
        page = await client.read_events(filter=event_filter, limit=5)

        page_count = 0
        while page:
            all_events.extend(page.events)
            page_count += 1
            page = await page.get_next_page(client)

        print(f"collected {len(all_events)} events across {page_count} pages")


if __name__ == "__main__":
    asyncio.run(query_recent_events())

Create, read, or delete artifacts

Create, read, or delete artifacts programmatically through the Prefect REST API. With the Artifacts API, you can automate the creation and management of artifacts as part of your workflow. For example, to read the five most recently created Markdown, table, and link artifacts, you can run the following:

fixture:mock_post_200

import requests


PREFECT_API_URL="https://api.prefect.cloud/api/accounts/abc/workspaces/xyz"
PREFECT_API_KEY="pnu_ghijk"
data = {
    "sort": "CREATED_DESC",
    "limit": 5,
    "artifacts": {
        "key": {
            "exists_": True
        }
    }
}

headers = {"Authorization": f"Bearer {PREFECT_API_KEY}"}
endpoint = f"{PREFECT_API_URL}/artifacts/filter"

response = requests.post(endpoint, headers=headers, json=data)
assert response.status_code == 200
for artifact in response.json():
    print(artifact)

If you don’t specify a key or that a key must exist, you will also return results, which are a type of key-less artifact. See the Prefect REST API documentation on artifacts for more information.

Workflows

Deployments

Automations

Workflow Infrastructure

Platform

Extensibility

How to use and configure the API client

Overview

Getting a client

Configure custom headers

Setting custom headers

Examples

Reschedule late flow runs

Get the last `N` completed flow runs from your workspace

Transition all running flows to cancelled through the Client

Create, read, or delete artifacts

​Overview

​Getting a client

​Pagination and default query limits

​Configure custom headers

​Setting custom headers

​Examples

​Reschedule late flow runs

​Get the last N completed flow runs from your workspace

​Transition all running flows to cancelled through the Client

​Query events with pagination

​Create, read, or delete artifacts

Overview

Getting a client

Pagination and default query limits

Configure custom headers

Setting custom headers

Examples

Reschedule late flow runs

Get the last `N` completed flow runs from your workspace

Transition all running flows to cancelled through the Client

Query events with pagination

Create, read, or delete artifacts